1. Field of the Invention
The invention relates to a tracking control circuit for making a light beam follow an information recording track on wobbling-track-included optical recording media, typified by CD-R, DVD-R, and the like.
2. Description of the Related Art
Conventionally, in optical information recording devices for recording information on an optical recording medium such as CD-R, it is advantageous, in terms of securing output power required of a recording light beam for information recording, for the tracking control on a recording track to adopt single beam tracking in which the light beam emitted from a pickup""s semiconductor laser is used with high efficiency. The single beam tracking commonly uses a push-pull method.
In single beam tracking control using a push-pull method, an information recording track formed on an optical disk is irradiated with the light beam output from the pickup. The reflected beam therefrom is received by a light receiving element which is divided in two by a dividing line optically parallel to a tangent direction of the information recording track. The difference between light reception outputs from the individual divided light receiving sections on the light receiving element is obtained to generate a tracking error signal. Then, in accordance with this tracking error signal, an objective lens is appropriately driven in the radial direction of the disk, i.e. in the direction orthogonal to the tangent direction of the information recording track, so that the light beam emitted from the semiconductor laser is controlled to stay on the information recording track.
The single beam tracking control using the push-pull method described above, however, involves a problem that a displacement between the optical axis of the objective lens and the optical axis of the light receiving element, produced by the exclusive driving of the objective lens in the disk radial direction can cause a tracking error even when the light beam is incident on the track center. More specifically, the above-mentioned displacement between the optical axes produces a direct current component (hereinafter, referred to as lens offset) in the tracking error signal, and hence the tracking control is made with a position biased by the amount equivalent to this lens offset as the target position (track center position).
The present invention has been achieved in view of the foregoing, and an object thereof is to provide a tracking control circuit which enables single beam tracking control independent of lens offsets.
The invention in a first aspect provides a tracking control circuit for making a light beam follow a wobbling track formed on an optical recording medium, comprising driving means for driving the light beam across the wobbling track under a tracking error signal generated in accordance with a reflected beam from the optical recording medium, of the light beam incident on the optical recording medium. The tracking control circuit further comprises: light receiving means for receiving the reflected beam, the light receiving means being divided into at least four sections by a first dividing line optically parallel to a tangent direction of the wobbling track and a second dividing line optically parallel to a direction orthogonal to the tangent direction; first computing means for obtaining a difference between the output signals from a first divided light receiving section and a third divided light receiving section located on a first diagonal line on the light receiving means, to output a first differential signal; second computing means for obtaining a difference between the output signals from a second divided light receiving section and a fourth divided light receiving section located on a second diagonal line on the light receiving means, to output a second differential signal; and phase comparing means for comparing the first differential signal and the second differential signal in phase to output a phase difference signal. Here, the phase difference signal is used as the tracking error signal.
According to the first aspect of the invention, the phase comparing means can compare the first differential signal and the second differential signal in phase to detect a phase difference which is proportional to the amount of off-track of the wobbling track. Therefore, driving an objective lens in accordance with a tracking error signal generated from the phase difference allows the tracking control to make a light beam follow a wobbling track with accuracy. Moreover, diffraction components caused by the wobbling are concentrated on the peripheral regions of the reflected beam received by the light receiving element. Accordingly, even if a lens offset occurs, no direct-current offset component will appear in the tracking error signal unless the displacement shifts the regions on which the wobbling-originated diffraction components are concentrated to over divided light receiving sections other than those for the regions to be received originally. This means no displacement of the target position.
In a second aspect, the invention provides a tracking control circuit for making a light beam follow a wobbling track formed on an optical recording medium, comprising driving means for driving the light beam across the wobbling track under a tracking error signal generated in accordance with a reflected beam from the optical recording medium, of the light beam incident on the optical recording medium. The tracking control circuit further comprises: light receiving means for receiving the reflected beam, the light receiving means being divided into at least four sections by a first dividing line optically parallel to a tangent direction of the wobbling track and a second dividing line optically parallel to a direction orthogonal to the tangent direction; first computing means for obtaining a difference between the output signals from a first divided light receiving section and a third divided light receiving section located on a first diagonal line on the light receiving means, to output a first differential signal; second computing means for obtaining a difference between the output signals from a second divided light receiving section and a fourth divided light receiving section located on a second diagonal line on the light receiving means, to output a second differential signal; phase comparing means for comparing the first differential signal and the second differential signal in phase to output a phase difference signal; third computing means for obtaining the sum of the output signals from the first divided light receiving section and the fourth divided light receiving section in the light receiving means to output a first sum signal; fourth computing means for obtaining the sum of the output signals from the second divided light receiving section and the third divided light receiving section in the light receiving means to output a second sum signal; fifth computing means for obtaining a difference between the first sum signal and the second sum signal to output a third differential signal; sixth computing means for obtaining a difference between the third differential signal and the phase difference signal to output a fourth differential signal; and seventh computing means for obtaining a difference between the third differential signal and the fourth differential signal to output a fifth differential signal. Here, the fifth differential signal is used as the tracking error signal.
According to the second aspect of the invention, the phase comparing means can compare the first differential signal and the second differential signal in phase to detect the amount of offset of the wobbling track. The amount of offset is subtracted from the third differential signal, or a push-pull error signal obtained by the fifth computing means, to generate the fourth differential signal. This fourth differential signal is subtracted from the third differential signal mentioned above to generate the fifth differential signal. Then, driving an objective lens with this fifth differential signal as the tracking error signal can remove a lens offset occurring in the push-pull error signal. This enables the tracking control of making a light beam follow a wobbling track with accuracy.